Let us face it! REST is the order of the day. And Atlassian just underlined that statement with the release of JIRA5.
With JIRA5, a lot of the operations can be done using the REST API and here is the full list of operations supported.
I thought I would attack this topic next given its growing popularity in the forums !. So why is this so popular? The context is pretty much given in the earlier tutorial for creating a SOAP client.
There are at least a couple of ways for extending the given, out of the box SOAP capabilities of JIRA. Those will be writing an RPC plugin (thanks to JIRA for giving that option) and re-building the RPC plugin.